The invention relates to tracing of signalling messages related to a subscriber in a mobile communication system, and particularly to tracing signalling messages of a specific subscriber.
Mobile communication systems have been developed because it has been necessary to be able to reach people even when they are not close to a fixed telephone terminal. The most important service provided by the mobile communication system is that the subscriber can make a call or he can be called, regardless of the subscriber's location in the coverage area of the system. Besides the mobile communication systems, the supplementary services provided through mobile stations have also developed. Most of these supplementary services are also available when the mobile subscriber roams from one network to another. To provide the services, the network elements or processes of the mobile communication system have to exchange information on the subscriber. This information is exchanged using signalling messages. Situations in which the subscriber cannot access the desired service or the service does not function as it should are relatively common. Causes of faults are searched for by tracing signalling messages related to the subscriber. In the pan-European mobile communication system GSM (Global System for Mobile Communications), for example, tracing is in practice carried out by taking snap shots from the signalling message flow and by analyzing their contents. A snap shot is a temporally limited sample from all signalling between the network elements or processes that are monitored.
The problem related to the arrangement described above is that as the signalling load increases, the buffers reserved for snap shots fill up rather quickly, and thus it is probable that the snap shots contain only some of the necessary messages or no necessary messages at all. Even if the signalling load were small, it would not be possible to ensure in any way that the snap shot contains the signalling messages that are to be traced. A further problem is that snap shots are usually rather large in size, and thus it takes a lot of time and effort to separate necessary messages from the unnecessary messages.